1. Field of the Invention
This invention relates to switching apparatus and more particularly to method and apparatus for maintaining a signal to a switching element conductive in response to current flow through a relay coil and thereby maintain the circuit in a "latched" state until current to the switching element is interrupted.
2. Description of the Prior Art In control systems for household appliances, such as microwave ovens and the like, switching or startup apparatus is used to allow the microcontroller to terminate rather than initiate current to the coil of a relay. With this arrangement the relay coil current is initiated by momentary closure of a "start" pad on a touch control panel to generate a latch trigger pulse to a switching element. Conventionally a relay controlled coil is connected in series with a contact pair. The switching element, such as a transistor, is placed in parallel with the contact pair. The trigger pulse is applied to the switching element by momentarily depressing the start pad on the keyboard of the appliance. The switching element upon receipt of the trigger pulse becomes conductive to provide current flow through the coil. As a result the contact pair in series with the coil closes. The coil current is then maintained by switching of the relay coil contacts to a "latched" state until the current to the coil is terminated.
With the above described prior art device a problem is encountered in depressing the "start" pad on the touch control panel for a shorter interval of time than required to close or "pull-in" the relay contacts to establish current flow through the relay coil. The "start" switch is depressed and released before a latched condition is established. Therefore, it is necessary to monitor the relay coil with an "on" sense signal to detect incomplete startup.
It is also well known with conventional latch circuits for solid state appliance controls that the presence of relatively small amounts of moisture can develop corrosion on the latching relay contacts. Corrosive deposits on the contacts prevent latching of the relay coil. Increasing the contact closing potential and gold plating the latching contacts are measures taken to overcome this corrosion problem. The apparent disadvantage of these corrective measures is the associated increase in cost of manufacture.
Therefore, there is need to improve the efficiency of switching apparatus for electronic latch circuits by eliminating the relay "on" sense signal to insure latching of the relay for preventing incomplete startup due to rapid operation of the start control. While attempts have been made to overcome the problems associated with corrosion bulldup on the contacts, more cost efficient measures are needed to increase the overall performance of the switching circuitry and reduce the circuit complexity.